UP1: Femtosecond X-ray diffractometer for time resolved X-ray diffraction experiments

Z. Ansari, F. Zamponi, Ph. Rothhardt, M. Bargheer, M. Wörner, T. Elsässer

Two laser-plasma based X-ray sources of copper Kα radiation are available in our group. One of these is a prototype (now available commercially) developed together with IfG GmbH on the basis of the first home-built setup. Both plasma X-ray sources have similar characteristics, generating bursts of copper Kα radiation with a yield up to 6 x 10¹⁰ photons/ second.

Several design improvements are incorporated in the new source. The interaction chamber is much smaller (10.5 × 4.5 × 6.5 cm³), and is made of aluminium to reduce the Bremsstrahlung caused by hot electrons. The copper tape target is passed over three rolls in the interaction chamber to keep it taut in the laser focus as small changes in the laser intensity on the target may give rise to substantial fluctuations in the X-ray yield.

Compared to last year, a larger beam diameter is employed to obtain tighter focussing of the laser beam on to the copper tape target. By using a 7.5 cm focal length parabolic mirror, a spot size of 2.5 microns FWHM can be obtained, corresponding to intensities on the order of 10¹⁸ W/cm² on the target. Addition-ally, a third-order autocorrelator was built and a commercial SPIDER system was bought to improve the diagnostics with an aim to suppress prepulses, crucial for efficient X-ray production. The new source is fully automated and the plasma source, the X-ray optics, the goniometer and the detector can all be controlled through an integrated software. Two detectors that measure the spectrum of the source and the X-ray yield facilitate optimizing the source for copper Kα production.

The home-built source is dedicated to the study of time resolved X-ray diffraction from single crystals. A cryostat allows for cooling samples cooled down to 4 K. The new source equipped with a large area CCD detector (6 cm x 6 cm), is applied to time resolved powder diffraction. The information obtained from both methods is complementary. While the angular resolution of the single crystal method and the strength of the obtainable signal are superior, the powder method enables one to study many different reflexes simultaneously and provides a simpler way to normalize the signal in order to be insensitive to fluctuations in the X-ray flux.

Fig. 1: Debye Scherrer rings of silicon obtained with the laser-plasma based femtosecond X-ray source.

The Debye-Scherrer method with sub-picosecond time resolution is an unexplored field in the realm of X-ray science. We first used our new set-up to study powder diffraction of silicon to check the feasibility of the method with such sources. Fig. 1 shows diffraction rings obtained for a silicon sample (38 images with an exposure time of one minute each were averaged). This preliminary study paves the way for time resolved femtosecond Debye-Scherrer studies.